The Mariner IV spacecraft, having achieved its mission objectives and now in its 300th day of flight, will receive a command from Earth next week, concluding--but possibly only temporarily--the National Aeronautics and Space Administration's longest and most complex deep space mission.

Since launch November 28, 1964, Mariner IV has transmitted to Earth nearly 50 million engineering and scientific measurements on the environment of interplanetary space and in the vicinity of Mars. It flew past Mars last July 14 at an altitude of 6118 miles, recording the first close-up pictures of the planet's surface.

After October 1, when the ground command switches the spacecraft transmitter from Mariner's high-gain directional antenna to an omnidirectional antenna, telemetry from Mariner IV will cease.

Although next week will mark the end of useful telemetry between Mariner IV and Earth during 1965, project officials at the Jet Propulsion Laboratory said the spacecraft will continue transmitting and may renew its radio link with Earth in 1967.

During the next two years, it will be possible to track Mariner IV only with a new 210-foot antenna, which will begin operation in April, 1966, at the Goldstone Space Communication Station in California. No telemetry will be received during this period, but periodic tracking of the spacecraft as it orbits the sun will determine whether or not its transmitter is still operating.

Tracking data during the long cruise will help in the evaluation of the new giant antenna system and hopefully will allow trajectory analysts to increase the accuracy of the known relative positions of Earth and Mars.

In June, 1967, Mariner IV and the Earth will be about 30 million miles apart, a distance over which communications can be resumed with the low-gain antenna. JPL engineers said several months of useful telemetry may be obtained at that time if the transmitter and other critical systems continue to operate.

Next Friday's antenna switchover command will be sent by a 100 kilowatt transmitter and beamed at the spacecraft from one of Goldstone's three 85-foot antennas. Some 17 minutes later, the command will reach Mariner, more than 191 million miles away. Another 17 minutes will pass before engineers in the operations center at JPL receive the final bits of information.

In addition to acting upon on-board commands from its central computer and sequencer, Mariner IV has received to date 84 ground commands from Goldstone and the four other stations of NASA's Deep Space Network, located in Johannesburg, South Africa; Madrid, Spain; and near Woomera and Canberra, both in Australia.

At 5 p.m. Pacific Daylight Time today, Mariner IV will be 187,152,860 miles from Earth and 17,606,438 miles from Mars. It has traveled more than 410 million miles in its orbit around the sun.